This invention relates to a wavelength-division multiplexing optical transmission system and repeater station in this system. More particularly, the invention relates to a wavelength-division multiplexing optical communication system and repeater station in which the capacity and transmission distance of an optical transmission system are increased by suppressing interchannel variations (inter-wavelength variations) in received light power, which are caused by wavelength-dependent gain of optical amplifiers and wavelength-dependent loss in the optical fiber of the transmission line.
Interchannel variations (inter-wavelength scattering) in the power of received light in a WDM optical amplifying repeater transmission system are caused by the characteristics and wavelength dependence of an optical lossy medium (optical devices, optical amplifiers and the optical transmission line) through which the wavelength-division multiplexed signal is transmitted, and may be classified into the following components depending upon the cause and characteristics:
1) a slope (primary slope) component caused by the wavelength-dependent loss of the optical transmission line and optical devices and by the Raman effect of the optical transmission line;
2) a beat component (a comparatively gentle rise and fall in the shape of the spectrum) caused by the wavelength-dependent gain of the optical amplifiers; and
3) a ripple component (a deviation on the order of 0.1 to 1 nm) caused by a gain equalizer in the optical amplifiers, an optical device used in an OADM (Optical Add/Drop Multiplexer), etc., a deviation in the output level of the transmitter in each channel and an adjustment error following wavelength-division multiplexing.
Interchannel variations in optical power at the receiving end that are the result of these factors produces variations in optical SNR and, as a result, degrades the transmission characteristic (bit error rate, or BER) and imposes a severe limitation upon the capacity and transmission distance of WDM optical transmission. More specifically, since the wavelength signal of lowest power among the multiplexed wavelength signals is the lower-limit value of receive power after transmission, the maximum transmission distance is limited by the wavelength signal of lowest power. Accordingly, reducing the variation between wavelengths after transmission is critical in terms of enlarging the maximum relay transmission distance.
To achieve this, flattening control and optical pre-emphasis control are available. Flattening control eliminates slope components of the optical spectrum with respect to wavelength (primary slope of the wavelength characteristic) and beat components by changing the excitation ratio of the excitation light of a Raman amplifier. Optical pre-emphasis control equalizes optical SNR (Signal-to-Noise Ratio) at the receiving end by changing the optical output level at the transmitting end channel by channel.
Flattening control by a Raman amplifier seeks to equalize, as much as possible, the input levels to optical amplifiers at the system nodes (optical repeater stations), and the object of pre-emphasis control is to calculate or measure optical SNR of each channel, adjust the output levels at the transmitting end and equalize the optical SNRs at the receiving end. By exercising such control, it is possible to improve the optical SNR characteristic of the overall system and achieve transmission of greater capacity and over longer distances.
FIG. 14 is a diagram useful in describing optical pre-emphasis control (see the specification of Japanese Patent Application Laid-Open No. 2001-203414). A WDM optical signal generated by an optical transmitter 11a in an optical transmitting station 11 is amplified by a plurality of optical repeaters 13a, 13b, . . . 13n, which are provided in optical transmission lines 12, so as to compensate for loss along the optical transmission lines 12 and loss in the optical repeaters 13a, 13b, . . . 13n, the amplified signal is transmitted to an optical receiving station 14 and the signal is received and processed by an optical receiver 14a. Loss in the optical repeaters 13a, 13b, . . . 13n is produced by optical component parts such as a dispersion compensating fiber used in the stations.
When the WDM optical signal is sent from the optical transmitter 11a to the optical transmission lines 12, pre-emphasis is applied by a pre-emphasis control circuit 11b within the optical transmitting station. That is, the pre-emphasis control circuit 11b calculates the difference between an average value of optical SNRs of all channels received from the optical receiving station 14 and the optical SNR of each individual channel and adjusts the optical level of each channel by an optical attenuator 11c so as to compensate for this difference. The optical transmitter 11a wavelength-division multiplexes the adjusted optical signals of all channels and sends the multiplexed signals to the optical transmission lines 12. The optical SNR of the optical signal of each wavelength is measured by an optical-SNR measurement circuit 14b provided in the optical receiving station 14, the information concerning the SNR is transmitted to the optical transmitting station 11 via a line 15 and then the above-described pre-emphasis control is repeated. As a result of the above operation, control is exercised in the optical receiving station 14 so as to uniformalize SNR.
FIG. 15 is a block diagram illustrating flattening control by a Raman amplifier.
A Raman amplifier produces gain in a signal wavelength that has been shifted from the wavelength of the excitation light by the amount of the Raman shift in the amplifying medium, as shown in FIG. 16. The amount of Raman shift and the Raman band are specific to the amplifying medium. Accordingly, if the excitation wavelength is shifted to the long-wavelength side, then the center wavelength of the gain and the gain band will be shifted toward the long-wavelength side by an amount identical with the amount of shift of the excitation wavelength. Further, optical amplification over a wide band is possible, as shown in FIG. 17, by inputting excitation light sources, which have slightly different excitation wavelengths from one another, to the amplifying medium collectively. Further, since gain varies in such a manner that the higher the power of wavelength of the excitation light, the greater the gain, any gain characteristic can be assigned to a Raman amplifier by controlling the power of each excitation wavelength (see the specification of Japanese Patent Application Laid-Open No. 2002-72262).
A plurality of optical signals (WDM signal light) are wavelength-division multiplexed and input to a back-excited Raman amplifying medium 21 from the input side of a Raman amplifier 20. A wavelength-division multiplexer 22 multiplexes excitation light of wavelengths λp1 to λp3 from excitation light-source blocks 23a, 23b, 23c, respectively, having different center wavelengths, and inputs the multiplexed signal to a combining coupler 24. The latter combines the excitation light of wavelengths λp1 to λp3 and the wavelength-multiplexed main-signal light, and supplies the combined signal to the Raman amplifying medium 21. A spectrum analyzer 25 detects the spectrum at the input section or output section (the input section in FIG. 15) of an optical amplifier 26 and inputs the detected spectrum to an excitation light controller 27. The latter calculates the slope (tilt) of the wavelength characteristic from the output of the spectrum analyzer 25, calculates the power of each excitation light signal so as to obtain a characteristic that will be the inverse of this wavelength characteristic and inputs the power to the excitation light-source blocks 23a, 23b, 23c. As a result, the excitation light-source blocks 23a, 23b, 23c generate excitation light of the wavelengths λp1 to λp3 having an intensity (excitation ratio) conforming to the input power, correct the tilt that is generated in the optical transmission line in the interval that undergoes compensation, flattens the wavelength characteristic and inputs the flattened characteristic to the optical amplifier 26.
Interchannel optical power variations and optical SNR deviations at the receiving end are minimized and high-capacity, long-haul transmission is made possible by the compensating scheme described above.
A further technique is to provide optical attenuators, the degree of attenuation of which can be varied, between a plurality of optical amplifiers disposed in an optical transmission line and flatten the wavelength characteristic by these optical attenuators (see the specification of Japanese Patent Application Laid-Open No. 2002-84024).
Flattening control by a Raman amplifier is extremely effective as a method of eliminating slope and beat components of a spectrum beforehand so long as the amount of attenuation applied by the optical attenuator of each channel disposed at the transmitting end in order to carry out optical pre-emphasis control has sufficient margin.
However, whereas the goal of flattening control by a Raman amplifier is to equalize input/output power of optical amplifiers, the goal of optical pre-emphasis is to equalize optical SNR. In view of this fact, it is necessary to consider items {circle around (1)} to {circle around (3)} below when both types of control are used conjointly.
{circle around (1)} If the input spectrum to the optical amplifier at each node is flat, then, theoretically, the optical SNRs at the receiving end should be uniform. However, since a Raman amplifier performs flattening one to several excitation light signals, there is a limit to flattening, beat components cannot be eliminated and neither can ripple components that produce a deviation in level from channel to channel. Consequently, in order to eventually equalize optical SNRs, it is necessary to carry out optical pre-emphasis control after flattening.
{circle around (2)} In a case where a wavelength-division multiplexing optical transmission system is introduced, usually such a system is introduced initially starting from a small number of wavelengths even though the system has a large capacity and a function capable of supporting a large number of wavelengths. Flattening control is control that adjusts excitation light so as to flatten the spectrum while observing the results of measurement by a spectrum analyzer at each node. However, it is highly likely that the excitation state which prevails when flattening is performed with a small number of wavelengths at the time of initial system introduction will differ from that which will eventually prevail when flattening is performed after the addition of a large number of wavelengths. In order to exercise optimum flattening control for a number of wavelengths and for every provided wavelength in such a manner that optical pre-emphasis control will be subjected to as little load as possible, it is preferred that flattening control be carried out when wavelengths are added on and when wavelengths are removed.
{circle around (3)} Thus, in order to eventually bring about the optimum state in terms of optical characteristics by pre-emphasis control, it is necessary to perform control in a certain order, namely flattening control first and then optical pre-emphasis control. It should be noted that after wavelengths are added on or removed, it is necessary that flattening control be performed again in order to lighten the load on pre-emphasis control. At such time there is the possibility that the condition of the spectrum that was optimized by pre-emphasis control with regard to the already existing wavelengths will be upset. This is a cause of degradation of the optical signal after the start of service and can lead to error. In order to avoid such a situation, control must be exercised a particular way when flattening control is carried out after wavelengths are added on or removed. Specifically, it is necessary to perform control in such a manner that wavelengths subsequently added on are flattened as much as possible while the spectrum that prevails following the preceding pre-emphasis is maintained. However, such flattening control is not performed by the prior art described in the examples of the patent specifications cited above.